A&A: New planet challenges theories of planetary system formation

Astronomers from Queen Mary University of London have discovered the first planet known to orbit a small, cool star in the opposite direction to the star’s rotation, a phenomenon challenging planetary system formation, the journal Astronomy & Astrophysics Letters reports.

The report said the finding is prompting scientists to reconsider established theories.

The newly discovered exoplanet, GJ 3090 b, is similar in size to Neptune. Planets normally orbit their stars in the same direction as the stars rotate. This is because stars and their planetary systems form from the same rotating disc of gas and dust, with planets inheriting its direction of rotation. However, GJ 3090 b appears to defy this pattern, with the inclination of its orbit confirming that it moves in a retrograde, or opposite, direction.

Astronomers had previously suggested that such orbital anomalies could be caused by strong gravitational interactions. A massive object, such as a stellar companion or giant planet, could gradually alter the orbit of a smaller neighbouring planet through its gravitational pull. However, researchers carefully studied the GJ 3090 system using the high-precision NIRPS infrared spectrograph and found neither a massive stellar companion nor another giant planet capable of explaining the phenomenon.

The absence of an obvious cause for the unusual orbit has led scientists to a new hypothesis about the system’s origins. They suggest that early in its history, the star may have attracted a second, tilted disc of gas and dust from the surrounding interstellar environment. The planet could have formed from this material, inheriting its unusual direction of rotation. In this scenario, the exoplanet’s strange retrograde orbit is not the result of a cosmic catastrophe or gravitational struggle, but rather a kind of “fossil” preserving evidence of the unusual conditions under which the system formed.

GJ 3090 is a red dwarf, a class of small, cool stars that make up the vast majority of stars in our Galaxy. Astronomers plan to search for other planetary systems with similar architectures. This could help test the two-disc hypothesis and provide a better understanding of how strongly the interstellar environment influences the formation of young planetary systems.

By Bakhtiyar Abbasov